1. Field of the Invention
The invention relates to digital to analog converters, particularly to processor controlled systems for automatically calibrating digital to analog converters.
2. Description of the Prior Art
Digital to analog converters (DAC's) are widely utilized in the electronics industry to convert digital words into analog output signals representing the value of the digital word. At the present state of the art, it is quite feasible to manufacture accurate twelve bit or ten bit DAC's utilizing monolithic integrated circuit technology and/or hybrid integrated circuit technology. However, more accurate DAC's, (e.g., having thirteen bit accuracy or more) for example, sixteen bit DAC's, are much more expensive to manufacture. Careful internal thermal design of hybrid integrated circuit sixteen bit DAC's is necessary to minimize "superposition errors". Superposition errors arise from a number of causes, including thermal variations produced by quiescent power levels changing for different operating conditions in the integrated circuit structure and non-zero temperature coefficients or tracking of temperature coefficients of various components in the integrated circuit structure. Limitations on accuracy of present state of the art sixteen bit DAC's arise from errors in gain, offset voltages, and non-linearities in the four to six most significant bits. Presently, available sixteen bit DAC's include a large number of adjustment or "trimming" potentiometers which can be adjusted to calibrate the four most significant bits to compensate for errors in offset, linearity, and gain. A well known technique for performing such calibration involves electrically isolating a sixteen bit DAC from an electronic system in which it operates, and manually adjusting appropriate ones of the above mentioned potentiometers to adjust the output voltage of the DAC to produce zero voltage, as measured by a six and one-half digit digital volt meter when all the digital inputs to the DAC are logical "zeros". Then each of the four most significant bits is adjusted by manually adjusting others of the potentiometers to produce an output voltage difference corresponding to "one least significant bit" voltage differences between each of the four most significant bits and the respective sums of all less significant bits. Finally, the gain of the DAC is adjusted by applying all "ones" to the digital inputs of the DAC and adjusting a gain potentiometer to produce a predetermined full scale output voltage. The foregoing manual adjustment procedure has several serious shortcomings. First, the electronic system in which the DAC operates must be electrically disconnected or isolated from the DAC. This, of course, involves "down time" for the electronics system.
Accordingly, it is an object of the invention to provide a digital to analog converter which can be calibrated with minimum interruption of an electronics system in which the digital to analog converter is operatively connected.
A large number of adjustment potentiometers must be adjusted to accomplish accurate calibration of presently available high accuracy sixteen bit digital to analog converters. Further, expensive calibration equipment is required, such as a six and one-half digit digital volt meter costing approximately $4000. The average time for a skilled technician to calibrate present sixteen bit DAC's ranges from at least 10 minutes to well over half an hour.
It is another object of the invention to provide a method and apparatus for calibrating a digital to analog converter connected in an electronic system, which method and apparatus require a minimum number of manual adjustments and which require utilization of a minimum amount of low cost external equipment to accomplish the calibration procedure.
Presently available sixteen bit DAC's are very expensive, due to the high cost in fabricating devices with the proper thermal design, precision matching of integrated circuit components, and manufacture of high accuracy, low temperature coefficient bulk metal film resistors utilized in precision ladder resistor networks required for the four most significant bits. The cost of the numerous trim potentiometers is also quite high, as they may range in cost from $1 to more than $3 or $4.
It is therefore another object of the invention to provide a low cost digital to analog converter having a resolution of at least sixteen bits and having correspondingly high accuracy.
It is another object of the invention to provide an automatic digital to analog converter calibration system.
Another shortcoming of even the best of the presently available digital to analog converters having greater than approximately twelve bit accuracy is the fact that accuracy is lost due to "drift" of component characteristics with both time and temperature. At the present state of the art, a sixteen bit high accuracy DAC must be recalibrated at least every three months, providing that the temperature range to which the DAC is subjected varies only by less than 10.degree. C. Any time that the temperature varies by more than 10.degree. C., accuracy is lost and the DAC must be immediately recalibrated.
It is yet another object of the invention to provide a high accuracy digital to analog converter device which can operate continuously with high accuracy over a wide temperature range.
There are many applications for DAC's wherein it is highly necessary that the output voltage of the DAC be monotonic, that is, the output always increases as the value of the digital input word applied to the digital inputs of the DAC increases, and wherein the output always decreases as the value of the digital input word decreases. This condition is present if the DAC has appropriately small non-linearity errors and superposition errors. However, the gain accuracy of a DAC frequently is less important than the linearity accuracy or monotonicity thereof, due to the fact that DAC's are commonly utilized in applications wherein comparison of a test signal with a control signal is made; under such conditions, linearity or relative accuracy is of primary importance. Thus, in such applications, it is highly desirable that the linearity of the DAC can be quickly and conveniently calibrated and recalibrated without making any external adjustments, without using any external reference voltage source, and without electrically isolating the DAC from the system in which it normally operates. Those skilled in the art will appreciate that if a digital to analog converter appears in a digital loop (for example, in a numerically controlled machine tool), non-monotonicity of the DAC could cause a control processor to produce an erroneous result or enter into a diagnostic routine or an "infinite loop".
It is therefore an object of the invention to provide a digital to analog converter of low cost wherein monotonicity of linearity can be automatically established by recalibration without use of any external reference.